Typically, in electromagnetic flow meters for measuring the flow rate of a fluid that is electrically conductive, the flow rate of the fluid that flows within a measurement pipe is measured by providing a magnetic excitation electric current that alternatingly switches polarities to a magnetic excitation coil that is disposed so that the direction of the magnetic field that is produced is perpendicular to the direction of flow of the fluid that is flowing within the measurement pipe, to detect the electromotive force that is produced between a pair of electrodes that are disposed within the measurement pipe perpendicular to the magnetic field produced by the magnetic excitation coil, and sampling and performing signal processing after amplifying the electromotive force that is produced between the electrodes.
Conventionally, in a magnetic excitation circuit for providing a magnetic excitation electric current to a magnetic excitation coil, the ramp up of the excitation magnetic field at the time of switching the excitation magnetism polarity, has been hastened through performing, conventionally, a method wherein two power supplies, high voltage and low voltage, are prepared in advance, where the magnetic excitation is performed using the high voltage when ramping up the magnetic excitation electric current, and then performing magnetic excitation with the low voltage otherwise. See, for example, Japanese Unexamined Patent Application Publication No. 2006-170968.
FIG. 14 is a block diagram illustrating a configuration for a conventional electromagnetic flow meter.
In a power supply circuit 91 for this electromagnetic flow meter 9, after the commercial AC power ACIN has been rectified, it is converted to high-frequency through a switching controlling circuit and supplied to the primary side coil of a transformer, and is rectified by a rectifying circuit that is provided on the secondary side coil of the transformer, after which the rectified power is stabilized by a voltage regulator, to provide various DC voltages to a controlling circuit 92 and a magnetic excitation circuit 93.
FIG. 15 is a circuit diagram illustrating a conventional magnetic excitation circuit. FIG. 16 is a signal waveform diagram illustrating the operation of the conventional magnetic excitation circuit.
This magnetic excitation circuit 93 switches, through a power MOSFET or through switching elements Q1 through Q4 that are made from analog switches, the polarities of the magnetic excitation power supply voltages VexH and VexL that are supplied from the power supply circuit 91, and provides them to a magnetic excitation coil Lex.
The power supply voltages for magnetic excitation can be selected as either VexH or VexL, from the high voltage VexH (for example, 30 V) or a low voltage VexL that is lower than the voltage of VexH (for example, 15 V), by a switch SW being switched by a voltage switching signal EXD1 from a CPU (not shown) within the controlling circuit 92.
Moreover, when it comes to the magnetic excitation polarity applied to the magnetic excitation coil Lex, the magnetic excitation polarity is switched through Q1 and Q4, from among the switching elements Q1 through Q4, being turned ON or OFF synchronously as a positive polarity pair, and Q2 and Q3 being turned ON and OFF synchronously, in the opposite phase from the positive polarity pair, as a reverse polarity pair, through one of the polarity switching signals EXD1 or EXD2 from the CPU.
As illustrated in FIG. 16, putting the power supply voltage for magnetic excitation to the high voltage VexH immediately after switching the magnetic excitation polarity increases the speed with which the magnetic excitation electric current Iex ramps up, and then, after a specific amount of time has elapsed after switching the magnetic excitation polarity, reducing the power consumption by switching the power supply voltage for magnetic excitation to the low voltage of VexL prevents heating of the transistor Q5, made from a power MOSFET, in the constant current circuit CCS. Note that the FIG. 16 illustrates an example wherein the ratio between the high-voltage interval TH and the low-voltage interval TL of the power supply voltages for magnetic excitation is set to 50:50, regardless of the ramp up of the magnetic excitation electric current Iex.
Of the switching elements Q1 through Q4, during the positive interval TP when Q1 is ON, when the ground electropotential VexCOM is set to the reference level (0 V), then point B, that is, the source electropotential VB of Q1 will be as illustrated in FIG. 16, and the power supply voltage for magnetic excitation will be about 30 V during the high voltage VexH high-voltage interval TH, and about 15 V during the low voltage VexL low-voltage interval TL. This is also true for point C, that is, the source voltage VC for Q3, during the negative interval TN when Q3 is ON.
On the other hand, during the negative interval TN when Q2 is ON, point D, that is, the source electropotential VD for Q2, will be 0 V until the magnetic excitation current Iex goes up, and thereafter the magnetic excitation electric current limitation by the constant current circuit CCS (for example, constant at 100 mA) will operate so that the power supply voltage for magnetic excitation will change to between 15 and 30 V of the high voltage VexH during the high-voltage interval TH, and change to between 0 and 15 V of the low voltage VexL during the low-voltage interval TL. This is true also during the positive interval TP when Q4 is ON. Note that the central voltage of VD is changed by the value of the resistance of the magnetic excitation coil Lex.
Typically, in order to turn a power MOSFET ON (to make the drain-source resistance RDS essentially 0Ω), it has been necessary for the gate-source voltage VGS to be a voltage that is adequately high compared to the threshold value of several volts, and to be a voltage that has margin in relation to the absolute maximum rating for VGS (which is typically between 20 and 30 V), and so normally a VGS of about 10 V has been used.
On the other hand, in the conventional technology, illustrated in FIG. 15, the source electropotential of the individual power MOSFETs that structure the switching elements Q1 through Q4 will vary greatly due to the switching of the power supply voltage for magnetic excitation, as described above, that is, due to the switching between the high voltage VexH (30 V) and the low voltage VexL (15 V).
Consequently, in order to maintain VGS=10 V regardless of the power supply voltage for magnetic excitation, three insulated power supplies (10 V×3), insulated from the ground voltage VexCOM, are necessary.
That is, in FIG. 15, Q1 is driven by an insulated power supply VexG1 (10 V)/VexG1COM (0 V), by a photocoupler PC1 that is operated by the polarity switching signal EXD1, and Q3 is driven by an insulated power supply VexG2 (10 V)/VexG2COM (0 V), by a photocoupler PC3 that is operated by the polarity switching signal EXD2.
Moreover, in FIG. 15, Q2 is driven by an insulated power supply VexG3 (10 V)/VexG3COM (0 V), by a photocoupler PC2 that is operated by the polarity switching signal EXD2, and Q4 is driven by an insulated power supply VexG3 (10 V)/VexG3COM (0 V), by a photocoupler PC4 that is operated by the polarity switching signal EXD1.
Because of this, as illustrated in FIG. 14, it is necessary to provide the magnetic excitation circuit 93 in the power supply circuit 91 by producing these three insulated power supplies by increasing the number of secondary side coils in the transformer and the number of secondary side rectifying circuits, and thus there is a problem in that this makes cost increases in the power supply circuit 91 unavoidable.
The present invention was created to solve problems such as these, and an aspect thereof is to provide a magnetic excitation circuit technology that can drive a switching element for switching the magnetic excitation polarity without requiring an insulated power supply.